The present invention relates to a process for recycling coal fines and ash particles entrained in the reaction effluent from a fluidized bed reactor system for the gasification of coal back to the fluidized bed for further gasification. In particular, the present invention relates to a process for recycling coal fines and ash particles from a fluidized bed reactor system in which critical sections of the gasifier exit pipeline, the cyclone inlet pipeline and cyclones are water-jacketed to avoid adhesion and buildup of coal fines and ash particles that can result in the plugging and eventual shutdown of the gasification system.
In fluidized bed gasifiers, such as that illustrated in Patel, et al., U.S. Pat. No. 4,315,758, the teachings of which are incorporated herein by reference, a large amount of coal fines are entrained out of the fluidized bed along with the raw product gas that exits the gasifier at the top. In order to achieve high carbon conversion efficiencies, these coal fines have to be collected in cyclones and returned back to the fluidized bed for gasification. Generally, the cyclones, due to the high temperatures of the product gas, are refractory lined to avoid erosion problems and to protect the metal shell of the cyclones.
It has been found that some coal ash particles, present in the coal feed to the reactor, become entrained along with the coal fines and are also present in the raw product gas leaving the top of the gasifier. These ash particles contain high amounts of iron or calcium compounds that are sticky or soft at the temperatures encountered in the cyclone. Because of the impact of these particles on the inlet of the cyclone, the sticky ash particles adhere to the refractory lined interior surface of the cyclone and eventually buildup so that they obstruct the passage of gas and plug the cyclone resulting in a shutdown of the gasifier reactor system. Similar ash adhesion also occurs on surfaces of sharp turns or any other obstructions, such as areas of reactor size reduction, because of the ash particle-containing gases impacting directly on these surfaces, which are also typically refractory lined.
Ash studies have shown that the reason for the adhesion of the ash particles is due to the chemical nature of the ash, the temperature of the particles, and the direct impact of the ash particles on refractory lined surfaces. It has also been found that replacing the refractory liner with a plain metal surface does not avoid the problem of particles adhering to the surface as the metal surface rapidly reaches the same temperature as that of the gas.
The degree and amount of ash adhesion is a function of the nature of the ash contained in a particular coal. However, it has been found that coal having a high sulfur and, therefore, iron content have a greater tendency to adhere to the metal surfaces. The vast coal reserves of the eastern United States fall into this category. Therefore, for a fluidized bed gasification system to be able to successfully utilize these coals, a solution to the ash adhesion problem is required.
One method of solving the ash adhesive problem that has been proposed is to cool the entire stream of the raw product gas and the coal fines to a temperature below 1400.degree. F., at which temperature the ash particles present in the coal fines do not adhere to surfaces on impact. This is achieved by spraying water or any other coolant in the top of the gasifier system to cool the gas to the above temperature as it exits the gasifier and enters the cyclone system. However, this method entails a considerable loss in overall efficiency in the gasification system. Once the gas is cooled, all of the sensible heat present in the gas cannot be recovered in the heat recovery system. Also, all of the coal particles present in the gas stream are cooled to the same temperature and, when returned to the fluidized bed reactor for gasification, require heating by combustion of additional amounts of coal and oxygen, thereby increasing operating costs. From this background, the present invention was developed.